Electrolytic tilt sensors include devices that provide output signals proportional to the angle of tilt and/or the direction of tilt when included as part of an appropriate electrical circuit. Tilt sensors were originally developed for weapons delivery and aircraft navigation and are now used in applications such as oil drilling, construction laser systems, automotive wheel alignment, seismic and geophysical monitoring, virtual reality systems, and robotic manipulators.
Most conventional electrolytic tilt sensors generally comprise a housing, or envelope, made of a non-conductive material, such as glass. The envelope is partially filled with an electrolytic solution and encloses a plurality of electrodes, which are partially immersed in the electrolytic solution when the tilt sensor is in its upright (i.e., zero tilt or electrical null) position. One of the electrodes, typically a center electrode, is a common electrode, and the remaining electrodes are sensing electrodes, which are typically grouped in one or more pairs that define one or more distinct tilt axes in conjunction with the center common electrode.
As the tilt sensor is tilted with respect to the horizontal, each of the sensing electrodes becomes more or less immersed in the electrolytic solution while the surface of the solution remains level with reference to the horizontal. The increase or decrease in immersion results in a corresponding change in impedance between any one of the sensing electrodes and the common electrode. This impedance change is measured by an electrical circuit and correlated to a tilt angle and/or tilt direction, depending on the number of sensing electrodes and the type of electrical circuit being used.
A shortcoming of glass-enclosed tilt sensors is that they are relatively fragile due to their glass construction. Glass-enclosed sensors must be handled with care and protected in special containment packages. They are costly to manufacture and generally use precious metal electrodes. Moreover, glass enclosed sensors may not be suitable for certain applications where a tilt sensor having a more robust enclosure is required.
In addition to electrolytic tilt sensors having non-conductive envelopes, tilt sensors having partially metallic envelopes, such as those disclosed in U.S. Pat. No. 5,630,280 to Crossan, Jr. and German Patent Publication No. DE 40 25 184 A1 to Geisel, have been described. Generally, such tilt sensors have two or four sensing electrodes extending into a chamber defined by the envelope, which comprises a metallic containment vessel and a header made of a non-conductive material. The metallic containment vessel functions as the common electrode while the header supports the sensing electrodes and insulates them from the metallic containment vessel.
A shortcoming of the Crossan, Jr. tilt sensor is that the relatively large glass seal can be susceptible to cracking caused by rough handling, age, harsh environment, and the like, which would lead to failure of the tilt sensor due to leakage of the electrolytic solution. In addition, the interface between the dissimilar materials of the seal and the containmnent vessel may provide a less than desirable seal, particularly if the tilt sensor were exposed to elevated temperatures.
At elevated temperatures the pressure inside the tilt sensor increases due to expansion of the gas and vaporization of the electrolytic solution inside the tilt sensor. When the temperature becomes too high, vaporized solution can escape between the seal and the containment vessel. Loss of even only part of the electrolytic solution will detrimentally affect the operation of the sensor.
The Geisel tilt sensor has a compression-fit O-ring gasket located between the containment vessel and the insulating header. A shortcoming of the gasket is that it is subject to deterioration over time, which may eventually cause the electrolytic solution to leak from the sensor. The Geisel tilt sensor also may be susceptible to vapor leakage and pressure loss due to failure of the mechanical seal at elevated internal pressures resulting from exposure to elevated temperatures. In addition, a gasket-type seal requires the additional gasket component, which adds to the complexity, difficulty, and cost of manufacturing the tilt sensor. Moreover, the Geisel electrodes appear to be adhesively bonded to the insulating header. Such adhesive bonds would be susceptible to destruction by the solvents of the electrolyte, particularly at elevated operating temperatures. Furthermore, neither the Crossan, Jr. nor Geisel tilt sensor is operable unless the containmnent vessel functions as the common electrode.
Due to the shortcomings of the above-mentioned tilt sensors, there is a need for a tilt sensor having a robust envelope and a highly reliable seal between the header and the containment vessel. In addition, there is a need for a tilt sensor having a metallic envelope and a center common electrode extending into the chamber defined by the envelope.